But Joseph Boden of the University of Otago's Department of Psychological Medicine is urging a slow-and-steady approach, saying the negative effects marijuana can have are being ignored.

"Relatively few contributors have discussed either the harms of cannabis or potential risks of legalisation," Dr Boden wrote in the latest NZ Medical Journal, published Friday.

"Most contributions imply that cannabis is a relatively harmless drug, and that cannabis law change will only have beneficial consequences. We would argue that, on the basis of evidence generated by longitudinal studies based in New Zealand, both assumptions are incorrect."

Dr Boden and his late colleague and co-author David Fergusson say evidence from the Christchurch Health and Development Study and the Dunedin Multidisciplinary Health and Development Study show marijuana use can be linked to:

educational delay

welfare dependence

increased risks of psychotic symptoms

major depression

increased risks of motor vehicle accidents

increased risks of tobacco use

increased risks of other illicit drug use

and respiratory impairment.

The effects are strongest for those under 18, and "could not be explained by social demographic and contextual factors associated with cannabis use".

"This research clearly establishes the facts that: a) cannabis use is not harmless; b) it has its greatest impacts on young users who are vulnerable to the neuro-psychological effects of cannabis."

Vaping marijuana is on the rise overseas.
Photo credit: Getty

A recent Australian study found it only took one or two sessions of marijuana use to change the structure of a 14-year-old's brain. Teens who'd got high only once or twice had more grey matter than those who hadn't, but this isn't necessarily a good thing.

The researchers said at that age, the brain undergoes a "pruning" process, getting thinner as it "refines its synaptic connections".

"One possibility is they've actually disrupted that pruning process," said University of Vermont professor Hugh Garavan, who led the study.

But are their societal benefits?

Dr Boden and Prof Fergusson suggest cannabis' negative effects may have been "suppressed to some degree by its legal status".

Their proposal

"We would propose that the first-stage decriminalisation process is evaluated at regular intervals by assessing the prevalence of cannabis use and cannabis-related harms. If this evaluation shows that that the decriminalisation process reduces harms, the next stage would be to move towards further liberalisation of cannabis laws."

Dr Boden says if the public votes for legalisation, legislation needs to be in place to stop weed shops becoming as common as liquor outlets.

"Because of the likelihood that trade in legalised cannabis will be controlled by large business conglomerates, there is little reason to expect that legalised cannabis will be more heavily regulated than alcohol, which has been the subject of increasingly liberalised regulation over the past 50 years."

Because of the risk to developing minds, Dr Boden says rather than setting the minimum purchase age at 18, like alcohol, it should be 20.

"The most prudent course of action for New Zealand to follow is to develop policies which eliminate the adverse effects of prohibition while at the same time avoiding the possible adverse consequences of full legalisation."